How Au Outperforms Pt in the Catalytic Reduction of Methane Towards Ethane and Molecular Hydrogen

Abstract

Within the context of a “hydrogen economy”, it is paramount to guarantee a stable supply of molecular hydrogen to devices such as fuel cells. Besides, catalytic conversion of the environmentally harmful methane into ethane, which has a significantly lower Global Warming Potential, is an important endeavour. Herein we propose a novel proof-of-concept mechanism to accomplish both tasks simultaneously. We provide transition-state barriers and reaction Helmholtz free energies obtained from first-principles Density Functional Theory by taking account vibrations for $2\hbox {CH}_4(\hbox {g}) \rightarrow \hbox {C}_2\hbox {H}_6(\hbox {g}) + \hbox {H}_2(\hbox {g})$ to show that $\hbox {H}_2$ can be produced by subnanometer $\hbox {Pt}_{38}$ and $\hbox {Au}_{38}$ nanoparticles. The active sites for the reaction are located on different planes on the two nanoparticles, thus differentiating the working principle of the two metals. The complete cycle to reduce $\hbox {CH}_4$ can be performed on Au and Pt with similar efficiencies, but Au requires only half the working temperature of Pt. This sizable decrease of temperature can be traced back to several intermediate steps, in excellent agreement with previous experiments, but most crucially to the final one where the catalyst must be cleaned from H($\star$) to be able to restart the catalytic cycle. This highlights the importance of including in catalytic models the final cleaning steps. In addition, this case study provides guidelines to capitalize on finite-size effects for the design of new and more efficient nanoparticle catalysts.